1. Field of the Invention
This invention relates generally to the art of thin film device processing and fabrication. More specifically, the invention relates to the fabrication of Organic Light Emitting Diode based displays and other devices.
2. Related Art
Display and lighting systems based on LEDs (Light Emitting Diodes) have a variety of applications. Such display and lighting systems are designed by arranging a plurality of photo-electronic elements (“elements”) such as arrays of individual LEDs. LEDs that are based upon semiconductor technology have traditionally used inorganic materials, but recently, the organic LED (“OLED”) has come into vogue for certain applications. Examples of other elements/devices using organic materials include organic solar cells, organic transistors, organic detectors, and organic lasers. There are also a number of bio-technology applications such as biochips for DNA recognition, combinatorial synthesis, etc. which utilize organic materials.
An OLED is typically comprised of two or more thin at least partially conducting organic layers (e.g., an electrically conducting hole transporting polymer layer (HTLs) and an emissive polymer layer where the emissive polymer layer emits light) which are sandwiched between an anode and a cathode. Under an applied forward potential, the anode injects holes into the conducting polymer layer, while the cathode injects electrons into the emissive polymer layer. The injected holes and electrons each migrate toward the oppositely charged electrode and recombine to form an exciton in the emissive polymer layer. The exciton relaxes to a lower energy state by emission of radiation and in process, emits light.
Other organic devices, such as organic transistors, organic sensors, and color filters will also typically contain a conducting organic (polymer) layer and other organic layers. A number of these OLEDs or other organic devices can be arranged in a pattern over a substrate as for instance in display system. One way of patterning organic electronic devices over a substrate is to create pockets by photo-lithography and then utilize a process known as ink-jet printing. The use of a photo-resist layer to define pockets for inkjet printing is disclosed in published patent application Number US2002/0060518 A1 entitled “Organic Electroluminescent Device and Method of Manufacturing Thereof”. In ink-jet printing, polymer or organic solution is deposited by discharging droplets of the solution into the pockets from a print head. One common application of inkjet printing is the patterning of multi-color OLED pixels (such as RGB patterned pixels) in order to manufacture a color display.
But inkjet printing and other selective deposition techniques which fabricate polymer films for devices have some limitations. One limitation is in being able to achieve multi-layer or “hetero-structure” devices that have adjacent films that are soluble in the same type of solvents. This is because each polymer solution which is deposited remains soluble even after drying. When an additional organic layer is required to be fabricated over an existing layer, the existing layer can only be made of a material which will not be soluble under the same solvent being used to deposit the additional layer. Otherwise, existing layers will be degraded substantially or even dissolved.
Recent developments have shown that UV curable inks can be used to deposit dye pigments for printing posters and textiles (U.S. patent application No. 20020044188). UV curable inks are solutions which cure or dry into film under application of ultraviolet or other radiation. For spin-coating (rather than selective deposition such as inkjet printing) techniques, a recent publication has outlined the use of “cross-linked” polymers to make RGB displays. See “Multi-colour organic light-emitting displays by solution processing”; C. David Muller, Aurelie Falcou, Nina Reckefuss, Markus Rojahn, Valerie Wiederhirn, Paula Rudati, Holger Frohne, Oskar Nuyken, Heinrich Becker, Klaus Meerholz; Nature Volume 421, Pages 829-833 (20 Feb. 2003). A cross-linked (or “cross-linkable”) polymer is a polymer which has been modified by the addition of a chemical group which chemically reacts with the original polymer to create side-chains which can alter the polymer's properties. In this publication, the authors propose spin coating UV curable inks that are then cross-linked such that the resulting film becomes insoluble. The films are then patterned to create the colored displays. This suffers from the drawback that additional processing is required on the deposited films in order to pattern them.
Thus there is a need for patterned devices that have hetero-structures wherein additional layers may be added to existing layers without degrading the integrity of existing layers.